The Effect of Internal Temperature Gradients on Regenerator Matrix Performance

2006 ◽  
Vol 128 (10) ◽  
pp. 1060-1069 ◽  
Author(s):  
K. L. Engelbrecht ◽  
G. F. Nellis ◽  
S. A. Klein
Keyword(s):  
Heat Transfer ◽  
Correction Factor ◽  
Temperature Gradients ◽  
Fluid Temperature ◽  
Time Varying ◽  
Internal Temperature ◽  
One Dimensional ◽  
Degradation Factor ◽  
Wide Range ◽  
The Impact

Background. One-dimensional regenerator models treat the solid material as a lumped capacitance with negligible temperature gradients. Advanced regenerator geometries operating at low temperatures or active magnetic regenerators which use a liquid heat transfer fluid may have temperature gradients in the solid regenerator that significantly affect performance. It is advantageous to utilize a one-dimensional, or lumped, model of the regenerator that is coupled with a correction factor in order to account for the impact of the internal temperature gradients. Previous work relative to developing such a correction factor is shown here to be inadequate or only valid over a limited range of dimensionless conditions. Method of Approach. This paper describes a numerical model of a sphere subjected to a time varying fluid temperature (representing a passive process) or time varying internal heat generation induced by a magnetic field (representing an active magnetic process). The governing equations are nondimensionalized and the efficiency of the sphere is presented as a function of the Fourier number and Biot number. Results. An approximate correction (or degradation) factor is obtained based on these results that is valid over a wide range of dimensionless conditions and therefore useful to regenerator designers. The degradation factor correlation was developed for a sinusoidal variation in the fluid temperature, however, the same results can be applied to different functional forms of the time variation using the concept of an effective cycle time that is weighted by the magnitude of the driving temperature difference. Conclusions. The heat transfer degradation factor presented here can be applied to one-dimensional regenerator models in order to accurately account for the transient performance of a matrix with finite thermal conductivity. This degradation factor allows regenerator models to approximately account for internal temperature gradients without explicitly modeling them and therefore remain computationally efficient while improving the range of applicability and accuracy.

Computational Study of the Heat Transfer of the Buoyancy-Driven Rotating Cavity With Axial Throughflow of Cooling Air

2009 ◽  
Author(s):  
Zain Dweik ◽  
Roger Briley ◽  
Timothy Swafford ◽  
Barry Hunt
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Free Convection ◽  
Flat Plate ◽  
Computational Study ◽  
Operating Conditions ◽  
Temperature Gradients ◽  
Wide Range ◽  
Complete Set ◽  
The Impact

Buoyancy driven flows that occur in the inter-disk space of an axial compressor spool play a major role in projecting gas turbine engine life and performance. The Rayleigh-Benard-like flow structure developed under the influence of centrifugal buoyancy creates sharp temperature gradients at the rotating walls of the compressor hardware. These sharp temperature gradients greatly influence the running stresses inside the machine and therefore affecting its life. The objective of this work is to generate a complete set of computationally-derived Nusselt number correlations that will be used in conducting the conjugate heat transfer analyses. The impact of engine power condition (Idle, Highpower, and Shutdown) on the heat transfer of these rotating cavities is studied under the wide range of operating conditions encountered by realistic turbomachines. A computational analysis is performed using commercially available computational tools for grid generation (ICEM-CFD) and turbulent-flow simulation (CFX). A total of fifty steady CFD cases for two different cavity configurations were analyzed. The CFD computed results of these cases were used to generate a complete set of Nusselt number correlations for different cavity geometry (gap ratios), flow regimes (forced and free convection dominated regimes), and operating conditions (Rossby Number Ro, Rotational Rayleigh Number RaΩ, and axial Reynolds Number Rez). The CFD computed heat-transfer results revealed that, despite the complicated flow patterns inside these cavities, and despite the large variation in their geometry, the simple Nusselt number correlations for free convection from a vertical flat plate with constant temperature can be used to predict the global Nusselt number values for the buoyancy-dominated regime of all such cavities. Furthermore, the Nusselt number correlations for the laminar and turbulent forced convection over a flat plate can be used to predict the global Nusselt number values for the central-jet dominated regime.

Study on the Frequency Response Characteristics of Thermal Stress Induced by Multidimensional Fluid Temperature Fluctuation

2012 ◽  
Author(s):  
Cristian Santiago Perez T. ◽  
Naoto Kasahara
Keyword(s):  
Heat Transfer ◽  
Thermal Stress ◽  
Frequency Response ◽  
Temperature Fluctuation ◽  
Hot Spot ◽  
Heat Diffusion ◽  
Fluid Temperature ◽  
Dimensional Approach ◽  
One Dimensional ◽  
The Impact

A simplified one dimensional approach for predicting the thermal stress in structures subject to near wall fluid temperature fluctuations has been previously developed and published by the author Kasahara. The method predicted the thermal stress by calculating the frequency response, formulated by the product of the effective heat transfer and the effective thermal stress related to one-dimensional temperature gradient developed through the wall thickness of the structure. Although, currently adopted by the Japanese Society of Mechanical Engineers (JSME) guideline for calculating the thermal fatigue damage in structures, recent studies have highlighted the limitations of the one dimensional approach by showing the presences of multidimensional fluid temperature fluctuation in plane direction, increasing the need to extend the current analysis to more detailed multidimensional guideline. The aim of this research is to advance the theoretical knowledge and understanding of complex multidimensional phenomenon related to local thermal fluctuations within small localized area at the surface of the structure, referred to as “Hot Spot” which is observed to have important effects on the thermal stress phenomenon. Furthermore, the understanding of heat transfer processes in the structure, especially heat diffusion that is known to produce a thermal gradient and, therefore, thermal stress. Understanding the behavior of each heat transfer process in the Hot Spot and the relationship to the response in frequency has formed the bases for extending the current one-dimensional model. This paper presents the analytical results of the study and proposes an extended multidimensional model to understand the thermal stress in tee-junction due to fluid temperature fluctuation and the close relation with the frequency. The model is derived from the understanding of the phenomenon which has leaded to quantify the effect by introducing certain multidimensional factors to explain the impact of the multidimensional fluid temperature fluctuation.

scholarly journals Some numerical experiments on firn ventilation with heat transfer

1993 ◽  
Vol 18 ◽  
pp. 161-165 ◽  
Author(s):  
M.R. Albert
Keyword(s):  
Heat Transfer ◽  
Surface Pressure ◽  
Thermal Effects ◽  
Numerical Experiments ◽  
Temperature Gradients ◽  
Pressure Amplitude ◽  
Two Dimensional ◽  
One Dimensional ◽  
Thermal Signature ◽  
Spatially Varying

Preliminary estimates of the thermal signature of ventilation in polar firn are obtained from two-dimensional numerical calculations. The simulations show that spatially varying surface pressure can induce airflow velocities of 10−5m s−1at 1.5 m depth in uniform firn, and higher velocities closer to the surface. The two-dimensional heat-transfer results generally agree with our earlier one-dimensional conclusions that the thermal effects of ventilation tend to decrease the temperature gradient in the top portions of the pack. Field observations of ventilation through temperature measurements are most likely to be observed when the firn temperature at depths on the order of 10 m is close to the air temperature, since steep temperature gradients can mask the thermal effects of ventilation. Preliminary indications are that, as long as surface-pressure amplitude is sufficient to move the air about in the top tens of centimeters in the snow, the resulting temperature profile during ventilation is fairly insensitive to the frequency of the surface-pressure forcing for pressure frequencies in the range 0.1–10.0 Hz.

A Natural Convection Fin with a Solution-Determined Nonmonotonically Varying Heat Transfer Coefficient

1981 ◽  
Vol 103 (2) ◽  
pp. 218-225 ◽  
Author(s):  
E. M. Sparrow ◽  
S. Acharya
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Natural Convection ◽  
Transfer Coefficient ◽  
First Principles ◽  
Numerical Solutions ◽  
Flow Direction ◽  
Operating Conditions ◽  
Fluid Temperature ◽  
Wide Range

A conjugate conduction-convection analysis has been made for a vertical plate fin which exchanges heat with its fluid environment by natural convection. The analysis is based on a first-principles approach whereby the heat conduction equation for the fin is solved simultaneously with the conservation equations for mass, momentum, and energy in the fluid boundary layer adjacent to the fin. The natural convection heat transfer coefficient is not specified in advance but is one of the results of the numerical solutions. For a wide range of operating conditions, the local heat transfer coefficients were found not to decrease monotonically in the flow direction, as is usual. Rather, the coefficient decreased at first, attained a minimum, and then increased with increasing downstream distance. This behavior was attributed to an enhanced buoyancy resulting from an increase in the wall-to-fluid temperature difference along the streamwise direction. To supplement the first-principles analysis, results were also obtained from a simple adaptation of the conventional fin model.

Use of a Zener Approximation for Heat Transfer Analyses of Quasi One-Dimensional Welding Problems

2018 ◽  
Vol 941 ◽  
pp. 2313-2318
Author(s):  
Jerry E. Gould
Keyword(s):  
Heat Transfer ◽  
Numerical Solutions ◽  
One Dimensional ◽  
Copper Electrodes ◽  
Wide Range ◽  
Linear Friction ◽  
Diffusion Transfer ◽  
Cooling Behavior ◽  
Transfer Problems ◽  
Analytical Approaches

Most welding methods in use today involve heating and subsequent cooling of the substrates for joining. Not surprisingly, understanding of associated thermal cycles implicit with the various processes has been a key facet of welding research. While the tools are available for sophisticated numerical solutions, much insight can be gained from simplified analytical approaches. A wide range of joining technologies in use today can be addressed by nominal one-dimensional heat transfer analyses. These include, for example, resistance spot, flash-butt, and linear friction welding. In addressing heat transfer problems, the mathematical constructs for heat transfer are analogous to those for mass (diffusion) transfer. Not surprisingly, one dimensional heat transfer problems can be greatly simplified by adapting the Zener approximation from mass transfer. The work described here employs the Zener approximation to address the direct spot welding of aluminum to steel. The Zener approximation is used to understand heat flow progressively from the steel into the aluminum and finally the copper electrodes. The results are used to understand weld morphology and implicit cooling behavior

scholarly journals Contagion Processes on Time-Varying Networks with Homophily-Driven Group Interactions

Complexity ◽  
2019 ◽  
Vol 2019 ◽  
pp. 1-13 ◽  
Author(s):  
Li Ding ◽  
Ping Hu
Keyword(s):  
Simplicial Complex ◽  
Connectivity Pattern ◽  
Time Varying ◽  
Interaction Patterns ◽  
Epidemic Threshold ◽  
Group Interactions ◽  
Wide Range ◽  
The Impact ◽  
Many Body Interactions ◽  
Contagion Process

The complicated interaction patterns among heterogeneous individuals have a profound impact on the contagion process in the networks. In recent years, there has been increasing evidence for the emergence of many-body interactions between two or more nodes in a wide range of biological and social networks. To encode these multinode interactions explicitly, the simplicial complex is now a popular alternative to simple networks. Meanwhile, the time-varying network has been acknowledged as a key ingredient of the contagion process. In this paper, we consider the connectivity pattern of networks affected by the homophily effect associated with individual attributes and investigate the impact of homophily-driven group interactions on the contagion process in temporal networks. The simplicial complex modeling framework is adopted to capture stochastic interactions between passively selected nodes in the paradigm of activity-driven networks. We study the evolution of infection and the epidemic threshold of the contagion process by both analytical and numerical methods. Our results on statistical topological properties of instantaneous network may shed light on accurately characterizing the evolution curve of infection. Furthermore, we show the impact of the homophily-driven interaction pattern on the epidemic threshold, which generalizes the existing results on both the paradigmatic activity-driven network and the simplicial activity-driven network.

scholarly journals Mathematical Modeling of Heat Transfer in an Element of Combustible Plant Material When Exposed to Radiation from a Forest Fire

Safety ◽  
2019 ◽  
Vol 5 (3) ◽  
pp. 56 ◽  
Author(s):  
Nikolay Baranovskiy ◽  
Alena Demikhova
Keyword(s):  
Heat Transfer ◽  
Forest Fire ◽  
Forest Fires ◽  
Plant Material ◽  
Mathematical Physics ◽  
Burned Area ◽  
Herbaceous Plant ◽  
One Dimensional ◽  
Impact Prediction ◽  
The Impact

The last few decades have been characterized by an increase in the frequency and burned area of forest fires in many countries of the world. Needles, foliage, branches, and herbaceous plants are involved in burning during forest fires. Most forest fires are surface ones. The purpose of this study was to develop a mathematical model of heat transfer in an element of combustible plant material, namely, in the stem of a herbaceous plant, when exposed to radiation from a surface forest fire. Mathematically, the process of heat transfer in an element of combustible plant material was described by a system of non-stationary partial differential equations with corresponding initial and boundary conditions. The finite difference method was used to solve this system of equations in combination with a locally one-dimensional method for solving multidimensional tasks of mathematical physics. Temperature distributions were obtained as a result of modeling in a structurally inhomogeneous stem of a herbaceous plant for various scenarios of the impact of a forest fire. The results can be used to develop new systems for forest fire forecasting and their environmental impact prediction.

Thermal–Hydraulic Performance Analysis of a Convergent Double Pipe Heat Exchanger

2019 ◽  
Vol 141 (5) ◽  
Author(s):  
Ahmed T. Al-Sammarraie ◽  
Kambiz Vafai
Keyword(s):  
Heat Transfer ◽  
Heat Exchanger ◽  
Hydraulic Performance ◽  
Operating Conditions ◽  
Contraction Ratio ◽  
Wide Range ◽  
Prandtl Numbers ◽  
Double Pipe ◽  
The Impact ◽  
Pipe Heat

The present investigation proposes an innovative convergent double pipe heat exchanger (C-DPHE). A two-dimensional (2D) axisymmetric heat transfer model with counterflow is employed to analyze the thermal and hydraulic performance of this configuration numerically. The impact of convergence in the flow direction, using a wide range of contraction ratio (Cr), is explored. The effect of Reynolds and Prandtl numbers on the flow and heat transfer is addressed, as well. The model results were validated with available data from the literature, and an excellent agreement has been confirmed. In general, the findings of the present study indicate that increasing the contraction ratio increases heat transfer and pressure drop in the C-DPHE. Moreover, this configuration has a prominent and sustainable performance, compared to a conventional double pipe heat exchanger (DPHE), with an enhancement in heat transfer rate up to 32% and performance factor (PF) higher than one. Another appealing merit for the C-DPHE is that it is quite effective and functional at low Reynolds and high Prandtl numbers, respectively, since no high-operating pumping power is required. Further, the optimal operating conditions can be established utilizing the comprehensive information provided in this work.

Effect of Temperature Ratio on Jet Impingement Heat Transfer in Active Clearance Control Systems

2019 ◽  
Vol 141 (8) ◽  
Author(s):  
Riccardo Da Soghe ◽  
Cosimo Bianchini ◽  
Jacopo D’Errico ◽  
Lorenzo Tarchi
Keyword(s):  
Heat Transfer ◽  
Jet Impingement ◽  
Effect Of Temperature ◽  
Internal Cooling ◽  
Temperature Ratio ◽  
Present Contribution ◽  
Wide Range ◽  
Depth Analysis ◽  
The Impact ◽  
Clearance Control

Impinging jet arrays are typically used to cool several gas turbine parts. Some examples of such applications can be found in the internal cooling of high-pressure turbine airfoils or in the turbine blade tip clearances control of aero-engines. The effect of the wall-to-jets temperature ratio (TR) on heat transfer is generally neglected by the correlations available in the open literature. In the present contribution, the impact of the temperature ratio on the heat transfer for a real engine active clearance control system is analyzed by means of validated computational fluid dynamics (CFD) computations. At different jets Reynolds number and considering several impingement array arrangements, a wide range of target wall-to-jets temperature ratio is accounted for. Computational results prove that both local and averaged Nusselt numbers reduce with increasing. An in-depth analysis of the numerical data shows that the last mentioned evidence is motivated by both the heat transfer incurring between the spent coolant flow and the fresh jets and the variation of gas properties with temperature through the boundary layer. A scaling procedure, based on the TR power law, was proposed to estimate the Nusselt number at different wall temperature levels necessary to correct available open-literature correlations, typically developed with small temperature differences, for real engine applications.

ENTRANCE EFFECTS OF THERMAL CREEP ON FLUID HEATING IN RECTANGULAR MICROCHANNELS

2013 ◽  
Vol 24 (08) ◽  
pp. 1350054 ◽  
Author(s):  
ALI AMIRI-JAGHARGH ◽  
HAMID NIAZMAND ◽  
METIN RENKSIZBULUT
Keyword(s):  
Heat Transfer ◽  
Wall Temperature ◽  
Temperature Jump ◽  
Velocity Slip ◽  
Temperature Gradients ◽  
Thermal Creep ◽  
Constant Wall Temperature ◽  
Rectangular Microchannels ◽  
Aspect Ratios ◽  
Wide Range

The effects of thermal creep on the development of gaseous fluid flow and heat transfer in rectangular microchannels with constant wall temperature are investigated in the slip-flow regime. Thermal creep arises from tangential temperature gradients, which may be significant in the entrance region of channels, and affects the velocity and temperature fields particularly in low Reynolds number flows. In the present work, the Navier–Stokes and energy equations coupled with velocity-slip and temperature-jump conditions applied at the channel walls are solved numerically using a control-volume technique. Despite the constant wall temperature, tangential temperature gradients form in the gas layer adjacent to the wall due to the temperature-jump condition. The effects of slip/jump and thermal creep on the flow patterns and parameters are studied in detail for a wide range of channel aspect ratios and, Knudsen and Reynolds numbers. Furthermore, the effects of variable properties on velocity-slip and, friction and heat transfer coefficients are also examined.

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